I bought this cherry '01 400EX this fall. Excellent shape, all original except bars and K&N Filter + jet kit. This thing starts and runs like crazy when it is warm outside, and occasionally starts and runs great when it is cold outside. Recently riding in O deg. weather, ran great. Problem is, if it doesn't fire on the first roll over it floods and doesn't fire again. Is this the jetting causing this??? This is driving me nuts!!!

 

Even though it may be flooding chances are that the problem is actually from it being lean due to the air being denser from the colder temps.

Easiet thing to try is to take three good stabs at the throttle (the carb has an accel pump so it will put fuel into the engine) and then try to start it. If you were using the choke then try it with and with out etc.

If that doesnt do it for you next easy thing would be to put the air box cover back on if it was removed, and maybe even reinstall the oem air filter, and all of this is to richen it up by reducing the amount of air getting into the engine.

The right way to do it is to rejet the carb richer for the colder temps, and dont forget to do the needle, and most importanty the pilot/slow jet. The main most likely needs to be upped to but the pilot controls the starting etc.

good luck cause jetting can be fun.

 

When I had my 400ex an 2001 to be exact that thing HATED to start when the temp was below 30 deg! It was pretty much stock too so I think it is just the cold blooded nature of the beast! good luck

 

Its got more to do with the stock pilot/slow jet size than anything else.

I can take mine any time of the year hit the throttle wide open a few times hit the starter and it starts, every time.

And I removed the choke a long time ago too.

 

The carb is jetted lean. Change the pilot jet, stock is a 38. Put in a 40 or 42 and it will start up on the 1st try and it will also run a whole lot better. While you are in there change the mainjet to a 152 or 155 The quad was jetted very lean from the factory.

 

I have the same quad only with no engine mods. I have found mine does the same thing and it is crazy hard to try and get warmed up in cold temperatures. The only way to do it is stick to it and toggle the choke until it will run fine..and dont get one it while its still cold..good way to ruin the engine..

 

Thanks for the input - the Dynojet kit and K&N Filter were installed before I bought the machine, looks like from leftover parts that the main jet is now a 142 (stock is labeled 148). Needle was also replaced - not sure why the needle changes or why there is a main jet adapter, why wouldn't the Dynojet parts re-use the stock needle? I assume the number refers to diameter or flow rate at a set condition? Anyway, it runs awesome once started, I have no complaints. Would changing the pilot jet affect warm weather starting/running? If I need to change jets to get it to run year round I'll do that, but if there was an optimum setup I'd set it and leave it. Thanks again.

 

You can run the larger pilot year round, but the one that will give you the better compromise from winter to summer between the different size pilot jets is something you need to decide. A little leaner will reduce any chance of running rich during idle in the summer, and a little richer will reduce starting and idle problems in the winter.

For my set up I found a 50 pilot to be the best compromise (not the size I recomend for yours though)

As far as the K&N "kit" goes the needle is slightly thinner and has a different taper than the stock one, and I still cant figure the reason for the adapter on the main except for patent issues with a direct copy of the Keihin.

Mine had the K&N when I got it too, but I threw that stuff in the drawer (except for the needle) and just picked up some Keihin jets locally ($3-$4ea). Biggest reasons were that the jets made by the carb mfg cant be bad, and the sizes and numbers are not exact and when you talk with anyone about jet sizes you cant compare properly.

the choice is yours as either will work.